The present invention relates to a central differential operation restriction device control device and to a method of operation thereof, for a vehicle adapted for four wheel drive operation and incorporating a four wheel drive power transmission system, and more particularly relates to such a central differential operation restriction device control device and method of operation thereof for a vehicle such as an automobile incorporating such a four wheel drive power transmission system, said central differential operation restriction device control device and method providing control according to the discrepancy between the rotational speed of the rear vehicle wheels and the rotational speed of the front vehicle wheels.
The present invention has been described in Japanese Patent Applications Ser. Nos. 61-105473, 61-105475, 61-133065, 61-153172, and 61-230543 (1986), all of them having been filed by an applicant the same as the entity assigned or owed duty of assignment of the present patent application; and the present patent application hereby incorporates into itself by reference the text of said Japanese Patent Applications and the claims and the drawings thereof; copies are appended to the present application.
Nowadays a greatly increasing number of automotive vehicles are being constructed with four wheel drive transmission systems, because such four wheel drive operation, in which all four wheels of the vehicle are powered from its engine via its transmission, is very suitable for driving on poor or slippery road surfaces such as in mud or over bad ground, or upon roads covered with mud, snow, ice, or rain. In other words, four wheel drive operation provides a much higher degree of stability and drivability for the vehicle in conditions where the coefficient of friction between the wheels and the surface upon which the vehicle is riding is relatively low. Also, four wheel drive operation is beneficial for aiding with hill climbing characteristics and high speed stability characteristics. Therefore, the so called full time four wheel drive type of transmission, which remains always engaged to four wheel drive without any episodes of two wheel dring, is becoming more and more popular.
In such a four wheel drive transmission system for an automotive vehicle, it is usual to provide a center differential device for distributing rotational power between the front wheels of the vehicle and the rear wheels of the vehicle, as well as the per se conventional rear differential device that provides differential action between the two rear vehicle wheels and the also per se conventional front differential device that provides differential action between the two front vehicle wheels. Such a central or front-rear differential device is provided in order to provide a differential action between said front vehicle wheels (considered as a pair) and said rear vehicle wheels (also considered as a pair) when the vehicle is turning around a curve, in order to eliminate the possibility of the occurrence of the so called tight corner braking phenomenon created by the difference in the turning radiuses of the front wheels of the vehicle and the rear wheels thereof (and also for various other reasons). And such provision of such a central or front-rear differential device is effective for achieving this result. Further, it has been practiced to provide an automatic transmission system to a vehicle which is equipped with such a four wheel drive type transmission. Such a type of structure is disclosed, for example, in Japanese Patent Application Laying Open Publication Ser. No. 56-138020 (1981). Further, it is per se conventional to provide, to such a center differential device, a torque distribution control clutch such as a central differential control clutch, which serves for regulating the distribution of the drive torque produced by the engine of the vehicle between the rear wheels of the vehicle (taken as a combination) and the front wheels of the vehicle (taken as a combination). Such a type of construction is disclosed, for example, in Japanese Patent Application Laying Open Publication Serial No. 50-147027 (1975) and in Japanese Patent Application Laying Open Publication Serial No. 55-72420 (1980). And such a torque distribution control clutch such as a central differential control clutch is typically controlled by a hydraulically operated servo device, so that the engagement pressure of said torque distribution control clutch, i.e. the maximum amount of torque that said torque distribution control clutch can transmit, which defines the amount of torque redistribution which said torque distribution control clutch can provide between the rear wheels of the vehicle (taken as a combination) and the front wheels of the vehicle (taken as a combination), is regulated by the magnitude of an actuating hydraulic fluid pressure. And such an actuating hydraulic fluid pressure is typically provided by a control system such as a hybrid electrical/hydraulic control system which may include a microcomputer.
Such a torque distribution control clutch or central differential action restriction means is typically provided for the following reason. If even one of the vehicle wheels slips, which may well occur especially when the vehicle is being operated upon a bad road surface such as when it is raining, it is snowing, or when the road is muddy, drive power will be lost, whereupon there is the problem that because of the differential effect of the center differential device the drive power of all wheels will be reduced, and the so called trailblazability of the vehicle will be severely deteriorated. In, therefore, a four wheel drive device having a center differential device, such a differential restriction device is typically provided. A four wheel drive device has already been proposed, constructed so that, when the difference between the rear wheel revolution rate and the front wheel revolution rate is at least a certain value, that is, when one vehicle tire is slipping with respect to the road surface, the differential control clutch is engaged, and the rear wheels and front wheels are directly coupled, whereas at other times the differential control clutch is released and the center differential device is allowed to carry out a differential effect; this is described in Japanese Patent Laying Open Publication Sho 55-72420 (1980).
If the differential effect of the center differential device is disabled when the difference between the front wheel and rear wheel revolution rates is at least a certain value, then the trailblazability of the vehicle upon a bad road surface is improved by the provision of the four wheel drive state with the front and rear wheels directly coupled, but, with this approach, when the certain difference in rotation rate between the front and rear wheels is attained, the drive state of the vehicle suddenly changes to four wheel drive state with the combination of the front and the combination of the rear wheels being abruptly directly coupled, and starting off on bad road trailblazability cannot be carried out smoothly. Moreover, when starting off, with the front and rear wheels being maintained in the directly coupled state by the action of the differential restriction device, it might be considered to make the torque distribution between front and rear correspond to the weight distribution ratio between front and rear wheels. If, however, the weight distribution ratio varies between the front and rear wheels, then the tire radius also differs between front and rear wheels, and there will be a difference in the rate of rotation between front and rear wheels, and at this point the front and rear wheels will be directly coupled, whereupon a circulation torque, the so-called screw up torque, occurs, accompanied by power losses and vibration.